1. Field of the Invention
This invention relates to an electron emission apparatus comprising electron-emitting devices, an image-forming apparatus and a voltage application apparatus for applying a voltage between electrodes.
2. Related Background Art
Known electron emission apparatus include image-forming apparatus such as an electron-beam display panel realized by arranging in parallel an electron source substrate carrying thereon a large number of cold cathode electron-emitting devices, a metal back or transparent electrode for accelerating electrons emitted from the electron-emitting devices and an anode substrate provided with a fluorescent body and evacuating the inside. An image-forming apparatus comprising field emission type electron-emitting devices is described in I. Brodie, “Advanced technology: flat cold-cathode CRT's”, Information Display, 1/89, 17 (1989). An image-forming apparatus comprising surface conduction electron-emitting devices is disclosed in U.S. Pat. No. 5,066,883. A plane type electron-beam display panel can be made lightweight and have a large display screen as compared with currently popular cathode ray tubes (CRTs) and can provide brighter and higher quality images than any other plane type display panels such as plane type display panel using liquid crystal, plasma displays and electroluminescent displays.
FIG. 17 of the accompanying drawings schematically illustrates an electron-beam display panel as an example of image-forming apparatus comprising electron-emitting devices. Referring to FIG. 17, there is shown a vacuum envelope 48 comprising a rear plate 31 operating as electron source substrate, a face plate 47 operating as anode substrate, an outer frame 42, a glass substrate 41 supporting the rear plate. The vacuum envelope 48 contains therein electron-emitting devices 34, wiring electrodes 32 (scan electrodes) and 33 (signal electrodes) connected to the respective device electrodes. Otherwise, there are shown a glass substrate 46 of the face plate 47, a transparent electrode (anode) 44 and a fluorescent body (fluorescent film) 45. The scan electrodes 32 and the signal electrodes 33 are arranged rectangularly relative to each other to produce a wiring matrix.
The display panel displays an image when selected ones of the electron-emitting devices 34 located at the crossings of the matrix are driven to emit electrons by sequentially applying a given voltage to the scan electrodes 32 and the signal electrodes 33 and the fluorescent body 45 is irradiated with emitted electrons to produce bright spots at locations corresponding to the activated respective electron-emitting devices. A High voltage Hv is applied to the transparent electrode 44 in order to give it a high electric potential relative to the electron-emitting devices 34 and accelerate the emitted electrons so that the bright spots may emit light actively. The voltage applied to the transparent electrode 44 is between hundreds of several volts to tens of several kilovolts depending on the performance of the fluorescent body. Therefore, the rear plate 31 and the face plate 46 are separated from each other normally by a distance between a hundred micrometers and several millimeters in order to prevent dielectric breakdown of vacuum (electric discharges) from occurring due to the applied voltage.
While a transparent electrode is used as acceleration electrode in the above arrangement, alternatively the fluorescent body 45 may be formed directly on the glass substrate 46 and a metal back may be arranged thereon so that a high voltage may be applied to the latter in order to accelerate electrons.
FIGS. 18A and 18B of the accompanying drawings schematically illustrate two possible arrangements of fluorescent film that can be used for an electron-beam display panel. While the fluorescent film comprises only a single fluorescent body if the display panel is used for showing black and white pictures, it needs to comprise for displaying color pictures black conductive members 91 and fluorescent bodies 92, of which the former are referred to as black stripes (FIG. 18A) or a black matrix (FIG. 18B) depending on the arrangement of the fluorescent bodies. Black stripes or a black matrix are arranged for a color display panel in order to make mixing of the fluorescent bodies 92 of the three different primary colors less discriminable and weaken the adverse effect of reducing the contrast of displayed images of reflected external light by blackening the surrounding areas. While graphite is normally used as a principal ingredient of the black stripes, other conductive material having low light transmissivity and reflectivity may alternatively be used.
A precipitation or printing technique is suitably be used for applying a fluorescent material on the glass substrate regardless of black and white or color display. The metal back is provided in order to enhance the luminance of the display panel by causing the rays of light emitted from the fluorescent bodies and directed to the inside of the envelope to be mirror-reflected toward the face plate 47, to use it as an electrode for applying an accelerating voltage to electron beams and to protect the fluorescent bodies against damages that may be caused when negative ions generated inside the envelope collide with them. It is prepared by smoothing the inner surface of the fluorescent film (in an operation normally called “filming”) and depositing an Al film thereon after forming the fluorescent film.
A transparent electrode (not shown) may be formed on the face plate 47 facing the outer surface of the fluorescent film 45 (the side facing the glass substrate 46) in order to raise the conductiveness of the fluorescent film 45.
Care should be taken to accurately align each of color fluorescent bodies and the corresponding electron-emitting device for a color display.
When a plane type image-forming apparatus using electron beams is made to have a large display screen, structural members called spacers may be required to protect the envelope against the pressure difference between the internal vacuum and the external atmospheric pressure. When spacers are used, they can become electrically charged as some electrons emitted from the electron source at locations near the spacers and/or cations ionized by electrons collide with the spacers directly or after being reflected by the face plate. When the spacers are strongly charged, electrons emitted from the electron source can be deflected to show respective swerved trajectories and get to the target fluorescent bodies at improper spots to display a distorted image having an uneven brightness distribution.
Techniques for solving the problem of electrically charged spacers by causing a small electric current to flow through the spacers have been proposed (see, inter alia, Japanese Patent Applications Laid-Open Nos. 57-118355 and 61-124031). According to one of such techniques, an electrically highly resistive film is formed on the surface of each insulating spacer to make a slight electric current flow therethrough.
Meanwhile, in an image-forming apparatus of the type under consideration comprising an oppositely disposed positive electrode such as a metal back or a transparent electrode, a high voltage is advantageously applied thereto in order to accelerate electrons emitted from cold cathode electron-emitting devices of the electron source so that the fluorescent bodies are made to emit light to a maximal extent. Additionally, the distance separating the opposite electrode from the electron source should be made minimal to display images with an enhanced degree of resolution because otherwise the electron beams emitted from the electron source can be dispersed before they get to the target electrode depending on the type of the electron-emitting devices of the electron source.
Then, a strong electric field is produced between the opposite electrode and the electron source due to the high voltage to give rise to electric discharges that can destruct some of the electron-emitting devices 34 and/or electric currents that can intensively flow through part of the fluorescent bodies to make the display screen partly and irregularly emit light.
Thus, measures should be taken to reduce the frequency of electric discharges and/or prevent electric discharge destructions from taking place.
An electric discharge destruction can occur when a large electric current flows through certain spots of the electron source to generate heat that destructs the electron-emitting devices located there or instantaneously raise the voltage being applied to some of the electron-emitting devices to consequently destruct them.
Measures that can be taken to reduce the electric current leading to an electric discharge destruction may include the use of a limitter-resistor inserted in series as shown in FIG. 19. However, such a measure by turn gives rise to another problem when a large number of electron-emitting devices are arranged in rows and columns, for example in 500 rows and 1,000 columns, and connected to a matrix wiring system so that they are driven sequentially on a line by line basis in such a way that as many as 1,000 devices are activated simultaneously. Assume now that about 1,000 devices are activated and each of them generates an emission current of 5 μA. Then, the electric current flowing through the anodes fluctuates between 0 and 5 mA depending on the image being displayed. Thus, when a resistor of 1 MΩ is connected externally in series as shown in FIG. 19, a voltage drop of 0 to 5 kV can take place to give rise to an irregularity of as much as 50% to the brightness for the acceleration voltage of 10 kV.
Additionally, since a high voltage is applied between a pair of oppositely disposed plates, the electric charge that can be accumulated due to the capacitor effect of the display apparatus will be as much as 10−6 coulombs if the cathode and the anode have a surface area of 100 cm2 and are separated by a distance of 1 mm and the potential difference between them is equal to 10 kV. This means that an electric discharge of 1 μsec. will cause an electric current of 1 A to flow through a single spot in the display apparatus, which is sufficiently strong to destruct electron-emitting devices. Thus, the arrangement of an external resistor that is connected in series does not provide any satisfactory solution if it can dissolve the problem of uneven brightness.